The present invention relates to a Fiber Grating Optical Waveguide Device comprising an optical fiber with an optical filter, both consisting essentially of silica, whereby said optical filter has an area with a Bragg grating region and a process for preparing a Fiber Grating Optical Waveguide Device according to the invention.
Optical filters have numerous applications in optical communications and in particular, they can be used for providing wavelength selectivity and tuning in WDM (wavelength division multiplexing) and DWDM (dense wavelength division multiplexing) systems. For this purpose, transmission filters are based on grating structures. Fibre grating structures as defined in the field of optics and hence in the following context comprise inter alia FBG (fiber Bragg grating), SBG (slanted Bragg grating), LPG (long period grating) and mode converters offer near ideal filter response and high channel isolation. The specific xe2x80x9cgratingxe2x80x9d is selected according to the requirements in the specific case. Furthermore they can be used in various sensor applications.
In the case of gain equalization it is often preferred to use a Slanted Bragg Grating (SBG) because it exhibits the absence of back-reflection at the filtering wavelength. Furthermore, the use of optical isolators is not mandatory. The SBG is a standard fibre Bragg grating tilted during photoinscription with an angle between the grating fringes and the normal of the fibre axis. This SBG couples most of the fibre-guided mode into radiation modes or cladding modes in a counter-propagating direction. The envelope of couplings into the different cladding modes yields the filter shape. The envelope of couplings is defined by the specific fibre design. It is for example known from FR-A-9806904, to provide a photosensitive cladding to reduce the spectral width of the grating and a reduced photosensitivity in the core to decrease the back reflection, whereby the grating can be short (800 xcexcm) or chirped to suppress the modulation due to coupling into discrete modes. By chirping, varying period fringes along the grating length, or by shortening the grating, each discrete cladding mode is enlarged.
It is often preferred to obtain a high transmission spectrum of the filter. In order to raise the coupling into the different cladding modes by the maximum photoinduced refractive index variation of the fiber material, the length of the grating has to be increased or the chirp rate of the grating has to be decreased. However, both methods lead to the disadvantage, that the discrete number of cladding modes induce unwanted discrete unitary filters in the transmission spectrum.
The underlying problem of the invention is therefore to avoid the disadvantages of the filters in the state of the art and to provide a fiber grating optical waveguide device which allows the light to be coupled into a continuum of radiation modes.
The problem underlying the invention is solved by a fiber grating filter optical waveguide device, comprising an optical fiber with an optical filter, both consisting essentially of silica, whereby said optical filter has an area with a grating region, wherein said area with a grating region is covered with a layer comprising a material having a refractive index which is the same or higher than the refractive index of the material of the optical fiber.
This leads to a layer of a material around the optical filter, which matches the refractive index in the cladding of the optical fiber. Thereby the transmitted light is coupled into a continuum of radiation modes.
In a preferred embodiment, the optically transmissive material smoothens the transmission spectrum of the filter.
Advantageously, the optically transmissive material is made from a monomeric and/or oligomeric precursor material, thus allowing to use a great variety of potential precursor materials selected according to the specific requirements. Furthermore, these precursors are generally more easily soluble in a variety of solvents than for example polymeric or amorphous materials.
In a further preferred embodiment, the precursor material contains hydrolyzable functional groups. Therefore easy accessible and cheap water-based solution can be used.
It is further preferred that the precursor material and the optically transmissive material contain at least one element selected from Si, Al, Ti, Sn, Ni and/or mixtures thereof.
In an advantageous embodiment, the precursor material contains organic groups, thus allowing to change, to vary and to determine in a facile manner the physico-chemical and therefore the optical properties of the final layer by respective selection of the organic groups R. Organic groups as understood within the scope of the invention consists mainly of C and H atoms, but may also include groups and moieties with heteroatom functionalities like N, S, P, Si, As etc.
In a further advantageous embodiment, the material is essentially attached via covalent bonds on the surface of the grating region. Therefore, the resulting layer is solidly fixed on the fiber surface and ensures furthermore a stable behaviour of the whole filter without filter deformation induced by mechanical stress.
It is preferred that the grating of the grating region is a Slanted Bragg grating, which is particularly advantageous, for gain equalization applications (small back reflection).
It is advantageous that the optical fiber has a core region and an outer region, whereby the core region displays a lower photosensitivity with respect to the outer region to decrease the back reflection.
The problem of the invention is further solved by a process for fabricating an optical device according to the invention comprising the following steps:
(a) Preparing a solution of precursor species of the material
(b) Bringing in contact the solution with the surface of the optical fiber comprising the grating region.
(c) Drying and curing of the optical fiber.
Bringing in contact according to step b) means for example dipping or spraying, or other methods known by a person skilled in the art. The bringing in contact occurs advantageously at room temperature, but higher or lower temeperatures work as well.
It is further preferred to clean the fiber surface before bringing in contact with the solution.
Advantageously, the curing takes place in a temperature range of from 60xc2x0 to 120xc2x0 C.
It is understood that the aforementioned advantages and the features of the invention explained in the following, are not only used in the specifically described combination, but can also be used by a person skilled in the art in other combinations or alone, without exceeding the scope of the invention.